Launch Creader Professional 909X - OBD II Monitors Explained

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LAUNCH Professional 909X User Manual
2.6 OBD II Monitors
An important part of a vehicle’s OBD II system is the Readiness Monitors, which
are indicators used to find out if all of the emissions components have been
evaluated by the OBD II system. They are running periodic tests on specific
systems and components to ensure that they are performing within allowable
limits.
Monitor operation is either “Continuous” or “Non-Continuous,” depending on the
specic monitor.
2.6.1 Continuous Monitors
Some of the vehicle components or systems are continuously tested by the
vehicle’s OBD II system, while others are tested only under specific vehicle
operating conditions. The continuously monitored components listed below are
always ready:
1. Misre Monitor
This Monitor continuously checks for engine misres. A misre occurs when the
air-fuel mixture in the cylinder does not ignite. The misre Monitor uses changes
in crankshaft speed to sense an engine misre. When a cylinder misres, it no
longer contributes to the speed of the engine, and engine speed decreases each
time the affected cylinder(s) misre. The misre Monitor is designed to sense
engine speed fluctuations and determine from which cylinder(s) the misfire is
coming, as well as how bad the misre is.
There are three types of engine misres, Types 1, 2, and 3.
• Type 1 and Type 3 misfires are two-trip monitor faults. If a fault is sensed
on the rst trip, the computer temporarily saves the fault in its memory as a
Pending Code. The MIL is not commanded on at this time. If the fault is found
again on the second trip, under similar conditions of engine speed, load and
temperature, the computer commands the MIL “On,” and the code is saved in
its long term memory.
• Type 2 misres are the most severe type of misre. When a Type 2 misre
is sensed on the rst trip, the computer commands the MIL to light when the
misre is sensed. If the computer determines that a Type 2 misre is severe,
and may cause catalytic converter damage, it commands the MIL to “ash”
once per second as soon as the misfire is sensed. When the misfire is no
longer present, the MIL reverts to steady “On” condition.
The Misfire Monitor is supported by both “spark ignition” vehicles and
“compression ignition” vehicles.